Aromatic compounds are conventionally produced from petroleum feedstocks in refineries by reacting mixtures of light hydrocarbons (C1-C6) and naphthas over various catalysts at high heat and pressure. The mixture of light hydrocarbons available to a refinery is diverse, and provides a mixture of aromatic compounds suitable for use in fuel once the carcinogenic benzene is removed. Alternatively, the hydrocarbon feedstocks can be purified into single components to produce a purer aromatic product. For example, aromatization of pure isooctene selectively forms p-xylene over some catalysts. The state of the art for production of 2,5-dimethyl-2,4-hexadiene (which can subsequently be dehydrocyclized to form p-xylene) typically uses isobutylene (obtained primarily from gas and oil crackers) and isobutyraldehyde (obtained by hydroformylation of propylene with syngas) as feed stocks (see FIG. 1).
Low molecular weight hydrocarbons can also be obtained by dehydration of low molecular weight alcohols produced via fermentation of biomass. For example, isobutanol can be dehydrated to provide isobutylene, a versatile 4 carbon synthon, which can then be dimerized under appropriate conditions to form 2,5-dimethylhexadiene.
However, conversion of low molecular weight alcohols (e.g., isobutanol) to 2,5-dimethylhexadiene may be difficult. For example, current methods for converting isobutylene (e.g., derived from isobutanol) to 2,5-dimethylhexadiene generally require high temperatures and oxygen co-feeds, which limit yields under typical conditions due to over oxidation of feedstock to carbon dioxide. Subsequent conversion of 2,5-dimethylhexadiene to p-xylene is typically a high yield, clean reaction over chromia (e.g., U.S. patent application Ser. No. 12/986,918, filed on Jan. 7, 2011, which is incorporated herein by reference in its entirety for all purposes) and other metal oxide catalysts. The reaction produces only p-xylene in the aromatic product and does so in high yield.
Further, using diisobutylene as a feedstock for the production of aromatic compounds (e.g., xylenes) generally results in some cracking (which affects yield) and the production of less desirable xylene isomers (e.g., o- and m-xylene) due to the presence of other and unavoidable isooctene isomers in the product stream from an isobutylene dimerization process.
The conversion of isobutylene and isobutyraldehyde over niobic oxide catalysts under acidic conditions to 2,5-dimethyl-2,4-hexadiene is known. Under similar conditions, t-butanol can be used in place of isobutylene since these conditions promote alcohol dehydration as well. For example, U.S. Pat. No. 4,684,758 employs catalysts which may include niobic acid to produce 2,5-dimethyl-2,4-hexadiene from blends ranging from 1-10:1 isobutylene/t-butanol to isobutyraldehyde. However, this process requires a discrete isobutyraldehyde feed for coupling with the isobutylene and/or t-butanol feed component(s). In addition, the isobutyraldehyde feed is conventionally prepared from other starting materials, such as propylene and formaldehyde, so the overall process entails multiple processing steps from multiple, different raw materials.